The invention lies in the field of optical metrology and related to optical coherence tomography (OCT). In particular, the invention relates to an apparatus and a method for the depth-dependent adaptation of the dynamic range of an OCT system to the profile of the backscattered power to be measured. The dynamic range of the measuring method can therefore be decoupled from the dynamic range of the analog/digital converter used. The invention is used, in particular, in the characterization of strongly scattering or strongly absorbing biological or technical samples.
Optical coherence tomography is a three-dimensionally imaging method which detects the optical scattering properties in the volume of a sample. OCT allows for spatial resolutions of a few micrometers in all spatial directions and is characterized by its high sensitivity. The areas of application of OCT are primarily in medicine and comprise, for example, tissue examinations in ophthalmology, dermatology, and dentistry. Furthermore, OCT is increasingly applied in materials science and process engineering. OCT systems exist in different embodiments. Widely used embodiments are the time-domain OCT (TD-OCT), first described by Huang et al. in “Optical Coherence Tomography,” Science, vol. 254, no. 5035, 1178-1181, November 1991; the spectral-domain OCT (SD-OCT), first described by Fercher et al. in “Measurement of intraocular distances by backscattering spectral interferometry,” Elsevier Optics Communications, vol. 117, 42-48, May 1995; and OCT with tunable laser source or swept-source OCT (SS-OCT), first described by Chinn et al. in “Optical coherence tomography using a frequency-tunable optical source,” Optics Letters, vol. 22, no. 5, 340-342, March 1997. The invention described herein relates particularly to the TD-OCT and SS-OCT embodiments, even though it is fundamentally not limited to said embodiments.
In OCT, the sample to be examined is irradiated with short-coherent light from a laterally single-mode source. A portion of the light backscattered from the sample is detected in laterally single-mode and brought to interference with the light of the source. The backscatter profile along the exciting light beam is determined with spatial resolution from the temporal coherence of the backscattered light and the emitted light. A three-dimensional depiction is generated by irradiating different points of the sample with the measuring beam and recording the various depth profiles of the backscattered power (so-called A-scans).
For example, a conventional OCT system consists of a Michelson interferometer, comprising an illumination arm, a sample arm, a reference arm, and a detection arm. Each arm can be an optical fiber or have a free beam optical design. The illumination arm contains a light source. The reference arm comprises an optical path and a reference reflector. The sample arm comprises an optical path and a focusing measuring head. The detection arm contains a photodetector. If a differential detection principle is applied, the illumination arm preferably performs the additional task of a second detection arm. For such purpose, the optical power is decoupled from the illumination arm to the differential detector by means of coupler/beam splitter or circulator.
For conventional TD-OCT, a broadband-emitting light source (e.g. white light) is used. Technical realizations, for example, can be superluminescence diodes (SLD) or short-pulse lasers in conjunction with high non-linear, e.g. microstructured, optical fibers. For detecting the coherence properties, the path length of the reference arm is modified by shifting the reference reflector. Backscattering along the measuring beam generates characteristic interference patterns which are recorded as function of the position of the reference reflector. This allows for the measurement of backscattering from different sample depths. A depth scan of the reflectivity profile thus obtained from a point in the sample is commonly called “A-scan.” The interference pattern on which the A-scan is based is available in the form of an analog electrical signal at the outlet of the photodetector. It is detected and digitally analyzed by an analog/digital converter (“AD converter”).
Similar to the TD-OCT, a conventional SD-OCT also has a broadband-emitting light source. However, the length of the reference arm for an SD-OCT is constant. The interference light is detected spectrally separate and the generating wavelength-dependent interference patterns are analyzed. For such purpose, an optical grid is frequently fitted on the detector side which images the different spectral portions onto a detector cell. The power on the individual detectors is read by means of an AD converter and further digitally processed. The depth-resolved backscatter intensity profile (A-scan) of the sample can be calculated by means of a numerical Fourier transformation.
The light source for conventional SS-OCT is a narrowband laser which is quickly adjusted over a large frequency band. The interference pattern is detected as a function of the optical frequency. The thus resulting analog electrical measuring signal is recorded after detection by means of analog/digital conversion and subjected to a numerical Fourier transformation. This results in a depth-resolved backscatter intensity profile (A-scan) of the sample.
The interferometric measuring method allows for highly sensitive, coherent detection of the light backscattered from the sample. In “In vivo ultrahigh-resolution optical coherence tomography,” Optics Letters, vol. 24, no. 17, 1221-1223, September 1999, Drexler et al. describe a time-domain OCT (TD-OCT) with differential detection, which is provided with a bandpass filter and an amplifier between detector and data acquisition. The filter is designed as a second-order Butterworth filter. It causes a suppression of noise components in the signal and thus allows for an increase of the measuring sensitivity.
In “Signal post processing in frequency domain OCT and OCM using a filter bank approach,” Photonics West, Proc. SPIE Int. Soc. Opt. Eng., page 644300-6, January 2007, Hofer at al. describe a frequency domain OCT system with a digital filter downstream of the detector array. The filter is used for compensation of the frequency-dependent decrease of the signal strength caused by the lateral expansion of the single detectors. The paper describes a numerical filtering downstream from an A/D converter.
However, in all these methods, particularly with strongly scattering or absorbing samples, the backscatter signal frequently decreases greatly with depth. The delimited dynamic range of conventional measuring systems thus also delimits the measurable depth. This, for example, is a problem when examining samples with a backscatter signal from the volume which is significantly weaker than the portion originating from the sample surface. Small signal portions which originate predominantly from deeper layers of the sample are no longer detectable due to a delimited dynamic range.
Therefore, the problem addressed by the invention is that of expanding the usable measuring depth of optical coherence tomography. This problem is solved by a system and a method for optical coherence tomography and a computer-implemented method and a computer system for controlling an optical coherence tomography measurement with the features in the independent claims. Preferred embodiments are subject matter of the dependent claims.